4 Land Magnetic Observations, 1905-10 



second distribution coefficient. The magnets are made of a special permanent 

 magnet steel manufactured by the Crucible Steel Company of America, and the 

 method of magnetization is substantially that given by Barus.^ Each magnet is 

 encased in a heavily gold-plated brass cylindrical sheath, which prevents rusting 

 of magnet and provides means for mounting the coUimating optical system, for 

 balancing, and for centering magnet in the stirrup by a projecting pin in the bottom 

 of the latter, the pin fitting snugly in a rectangular groove cut in the brass sheath. 

 The weight of the long magnet with sheath is the same as that of the short magnet 

 with its sheath. The reticle consists of two engraved intersecting lines at right 

 angles on a piece of piano-parallel glass mounted at the south end of the sheath; 

 hence, when using the magnetometer, the observer sights towards the magnetic 

 south. The magnet may be placed quickly in position, "erect," or "inverted" 

 with the aid of marks cut in the beveled edge of the stirrup. The entire suspension 

 system is constructed as simply as possible; the devices for clamping the phosphor- 

 bronze ribbon suspension have no removable parts and permit easy and rapid 

 insertion of a new ribbon. 



The ribbon now used ordinarily for the suspension is 0.010 by 0.127 mm. in 

 section; that for inertia determinations is of a somewhat heavier grade. At 

 Washington, where the horizontal intensity is about 0.20 C. G. S., the torsion for 

 the lighter ribbon, for the suspension-length used, amounts to about 5 minutes 

 of arc for 90 of twist. The total length of the suspension, i. e., the distance from 

 the bottom of the ribbon clamp at the suspension head to the center of the mag- 

 net when in place, is from 314 to 364 mm., depending upon the length of the ribbon 

 between the upper and lower clamps; ordinarily the total length is about 340 mm. 

 The graduated circle for the determination of the line of detorsion is permanently 

 attached to the stirrup and brought into the focus of the reading telescope by 

 interposing an auxiliary lens mounted at the telescope end of the magnetometer 

 house and operated by a milled head. The angle of detorsion from the axis of 

 the instrument may be quickly determined by noting the extreme swings on the 

 graduation of the stirrup when the brass torsion weight, equal in weight to the long 

 magnet as also of the short one, is suspended, and shifting the suspension head 

 accordingly; this operation is repeated until the mean reading on the graduation 

 of the stirrup is 180, when the head is clamped in position. 



The stirrup is of brass, heavily gold-plated to prevent oxidation, and of hollow 

 rectangular cross-section, the width being made equal to the diameter of the 

 magnet sheaths and the length sufficient to insure accurate orientation of the 

 magnets. The height of the stirrup is such as to permit the mounting of an inertia 

 bar, having the same diameter as that of the complete magnet, for the determi- 

 nation of the moment of inertia of the system. The stirrup may be easily raised 

 or lowered by means of a rack and pinion arrangement operated by a milled head 

 at the top of the suspension tube. As in the case of the magnetometer of the 

 Magnetic Survey of India, there is provided an arrester for quieting the motion 



Barus, Carl. Effects of hardness on the electrical and magnetic constants of steel, with particular reference 

 to the tempering of the magnetic parts of instruments. Terrestrial Magnetism, v. 2, 1897. (1-10.) Also, Bulletin 

 of the United States Geological Survey, No. 14, 



